Wouldn't the most important safety aspect of jumper cables be prevent a complete short, as BrianR did with the wrench?

That's highly recommended.:D

Originally, battery terminals were exposed. In mid-1970s, GM introduced Freedom battery with terminals on the side and mostly protected (more difficult to jump start). However Japanese created what is now defacto standard on cars - the plastic cap over + terminal. Installed just so the bouncing wrench does not cause battery explosion. Previously the bouncing wrench could result in you signing words usually known only to angels.

IN response to previous post:

BTW, hydrogen is a danger when the fan is moving - because that is when the battery is finally under strong recharge. A running engine is required to create enough current so that battery might release significant amounts of hydrogen.

When talking about current necessary to cause a battery explosion, then yes - those heavy cables from battery to car frame do restrict current flow making battery explosion less likely or slowing down the time to explosion.

True but with the fan running and probably the hood up the gas doesn't accumulate. A stream, even a strong one, could ignite and scare the hell out of you. But it would not explode and would not backtrack into the battery setting it off.
Even a dead short should not cause an explosion because of the internal resistance of the battery becomes current limiting. The danger is at the contact points of the short. Arcing causes heat, flame, molten metal, etc. A friend of mine arced a 24 volt system with a wrench in his left hand and lost his ring finger by melting his wedding ring. Another danger of marriage. :D

Battery explodes because internal battery resistance dissipates the entire short circuit. In a car battery, that resistance may be on the order of hundreths of ohms. In a short circuit, all battery electrical power would be dissipated by that internal resistor - resulting in a battery explosion.

However if shorted by a circuit that also includes a battery cable, then the short circuit would have more fractional ohms to also dissipate the power - outside the battery. Granted, that cable is still too low to provide full protection. But the additional resistance of battery cable will at least slow down the 'time to explosion'. Gives a human time to correct his mistake.

Returning to the original question - probably the most important reason to ground to chassis - one cannot confuse which is positive and negative. Therefore someone will not blow out car's entire electrical system. In can't tell you how many times the black electric cable was replaced by a red one. Mechanic was more concerned with what he had rather than doing it right. Then when someone jumpered the battery, they hooked positive cable to the red wire. Sparking is well after damage has been done.

The rest of the circuit is another battery and cables that are smaller in diameter (more resistance) and 8 to 10 times as long as the ground cable. Therefore I can't see the ground cable helping very much.

Most modern cars, trying to jump them backwards will just blow the main fuse and not damage anything else. Not that this is a good thing!

Gel and AGM batteries will outgas if mistreated -- normally they are sealed but there's normally a controlled weak spot in the enclosure which will blow out under overcharge.

Just a side thought.
I needed my pickup the other day so I started it for the first time since 3 days before the "BIG SNOW". February, wasn't it?. Anyway, start it and idle for 4 or 5 minutes, the gel battery was completely recovered according to the gage. Sweet.:D

Color of cables matter?
 

I've only heard that one needs to hook up the red cable to positive and the the black cable to negative. But if they're just cables, why couldn't you hook up the black cable to positive and the red cable to negative? So long as you're consistent with placing black on positive for both cars and red on negative for both cars, I don't see why red needs to be on positive and black on negative. Do the cables have some sort of diode to block current going in a particular direction?

A diode in that circuit would be immediately destroyed. It's true, as long as you're consistent and have the same hookup on both cars, the color doesn't matter. It's just convention and simpler to instruct.

I typically use the bumper mount on older cars and the towing point for newer cars for jump starting. You'll have to bend down to find the tow point, but it gives a good earth to the body!

Can I still put one on each nipple?

Has anybody EVER had a car start by following the POS-POS, NEG-Engine Block/frame?

Yes. :)

Yepper!